Short answer: Where the occupancy-specific data sheet gives no value, use the defaults in DS 2-0 Table 2.2.1.3.3 (Section 2.2.1.3.3, p.13). Wet systems: black steel C=120, polymer enhanced pipe 140, plastic 150. Dry and preaction systems: black steel C=100, but 120 where the gaseous medium is an inert gas such as nitrogen, 120 for internally galvanized pipe and 140 for polymer enhanced pipe. Refrigerated-area and vacuum systems have their own rows.

DS 2-0 does not let you read pipe size from a table. Section 2.4.1.1.1.2 (p.32) puts the rule in one sentence: whatever size is chosen, it must be hydraulically proven to meet the minimum design recommendations of the occupancy-specific data sheet. What DS 2-0 provides is not the pipe size but the boundary conditions of the calculation.

The source is FM Global DS 2-0, Installation Guidelines for Automatic Sprinklers (October 2021, Interim Revision April 2025, 187 pages). For installation and hangers see the DS 2-0 installation field guide and the DS 2-0 pipe and hanger detail; for friction tables see DS 2-89, and for the overall structure of the calculation see DS 3-0.

Which Data Sheet Governs

DS 2-0 does not present itself as a design source. Section 2.1.2.3 (p.10) sends you to the occupancy-specific data sheet for design criteria (sprinkler system design, hose stream allowance, duration) and states that its recommendations can supersede the general recommendations of DS 2-0. Sections 2.2.1.3.1 and 2.2.1.3.2 (p.12) repeat the split: the design comes from the relevant occupancy data sheet, the hydraulic calculation method from DS 3-0. Section 2.1.5 (p.11-12) makes the routing concrete: DS 3-26 for nonstorage areas, DS 8-9 for storage. Before sizing anything, fix in writing which data sheet governs.

Hazen-Williams C-Value

Where the occupancy data sheet does not give a C-value, the defaults of Section 2.2.1.3.3 (p.12) and Table 2.2.1.3.3 (p.13) apply:

System type Pipe and condition C-value
Wet Black steel 120
Wet Polymer enhanced / plastic 140 / 150
Dry or preaction Black steel 100
Dry or preaction Black steel, gaseous medium an inert gas such as nitrogen 120
Dry or preaction Internally galvanized / polymer enhanced 120 / 140
Refrigerated area Black steel 100
Refrigerated area Ambient always at or below 20 °F (-7 °C), or inert gas 120
Vacuum Black steel 100
Vacuum Conditions of Section 2.2.1.3.4 met, or ambient always at or below -7 °C 120

This table corrects the common site shortcut that "FM always wants C=100": for a wet steel system the default is 120. The drop to C=100 is specific to dry, preaction, refrigerated-area and vacuum systems, and using nitrogen brings that penalty back to 120. The nitrogen decision is therefore not only a corrosion decision but a direct input to the hydraulic calculation. Section 2.2.1.3.4 (p.13) lists five conditions for C=120 on a vacuum system; the first is single-path flow throughout the system, meaning no loops or grids.

Topology: Tree, Loop, Grid

Section 2.2.1.5.1 (p.14): the piping of wet-pipe and deluge systems can be a tree, loop or grid. Section 2.2.1.5.2: dry-pipe, preaction, refrigerated-area and vacuum systems are arranged for single-path flow (no loops or grids), the water supply must meet the maximum water delivery time, and Section 2.2.1.4 must be satisfied. Section 2.2.1.5.3 gives a narrow exception: the feed main and cross main of dry-pipe and preaction systems protecting a nonstorage area may be looped.

A grid also carries a maintenance cost. Section 2.2.1.7.2 (p.14) requires one end of each branch line on a grid system to be detachable by a simple union or flexible joint; FM Approved branch line test devices are an accepted alternative. Section 2.2.1.7.1 requires removable flushing connections at the ends of cross mains and of near and far mains on all systems, with a minimum size of 1.25 in. (32 mm). For a comparison of system types see wet, dry, alternate and preaction systems compared.

Coverage Area and Water Delivery Time

Section 2.2.1.4 (p.14) sets three limits. The maximum area of a wet-type ceiling-level system is limited only by the hydraulic requirements and the waterflow alarm recommendations of Section 2.3.3. The maximum area of a dry-type ceiling-level system is limited by the hydraulic requirements and the water delivery time stated in the design. The maximum area of an in-rack system (wet or dry) is 40,000 ft2 (3,715 m2) of floor area occupied by the racks, including aisles, regardless of the number of levels.

Where the occupancy data sheet gives no value, Section 2.2.1.11.1 (p.16) asks for one of these options: 60 seconds with the single most remote sprinkler operating; for nonstorage occupancies, 50 seconds with the most remote 2 sprinklers on the most remote branch line; for storage occupancies under obstructed ceiling construction, 40 seconds with the most remote 2 sprinklers on the most remote branch line; under unobstructed ceiling construction, 40 seconds with the most remote 2 sprinklers on each of the most remote 2 branch lines (4 in total). For deluge systems the maximum is 30 seconds where no value is given (Section 2.2.1.11.2).

For sizing, the key sentence is Section 2.2.1.11.3: experience has shown that reaching these times requires the system size to be limited to approximately 10,000 to 12,000 ft2 (930 to 1,115 m2). The same section asks for plans, calculations, water supply and equipment details to be submitted to the local FM office before work starts, and for the evaluation to include the dry valve make and model, the air pressure to be held in the piping, and computer calculations showing the times.

Pipe: Size, Wall Thickness, Pitch and Flushing

Minimum size. Section 2.4.1.1.1.1 (p.32): the minimum size for sprinkler system piping is 1 in. (25 mm).

Wall thickness. Table 2.4.2.2.1(2) (p.36) gives metric minimums by joining method: DN25-DN40, 2.6 mm for welded or roll-grooved pipe and 3.2 mm for threaded or cut-grooved pipe; DN50, 2.6 and 3.6 mm; DN80, 2.9 and 4.0 mm; DN100, 3.2 and 4.5 mm; DN150, 4.0 and 5.0 mm; DN200, 4.5 and 6.3 mm; DN300, 5.6 and 7.1 mm. A table note allows 1.7 mm for roll-grooved pipe when used with couplings or fittings listed in the Approval Guide for that thickness. Over water-sensitive occupancies (data centres, museums and semiconductor manufacturing are given as examples) Table 2.4.1.1.2.1(b) (p.33) asks for heavier walls: 3.2 mm for DN25-DN40, 4.5 mm for DN100, 5.0 mm for DN150 and 7.1 mm for DN300.

Pitch. Section 2.4.1.1.3.1 (p.34) requires dry-pipe and preaction piping to drain back to the system's main 2 in. (50 mm) riser drain: branch lines at least 1/2 in. per 10 ft (4 mm/m), all other piping at least 1/4 in. per 10 ft (2 mm/m). Auxiliary drains are provided for portions that cannot drain back (Section 2.4.1.1.3.2).

Bending. Section 2.4.1.1.4.1 (p.34): steel pipe with a wall thickness of at least 2.6 mm may be bent provided the bend radius is at least 12 pipe diameters and the pipe stays round. For a smaller radius, Table 2.4.1.1.4.2 (p.34) gives the minimum wall and allowable radius per size (for example 3.2 mm with 6 pipe diameters for DN25, 6.0 mm with 5 pipe diameters for DN100).

Flushing. Section 2.1.3.5 (p.10) requires all underground mains to be flushed until the water runs clear before connection; one of the acceptable minimum flows is the flow giving a velocity of 10 ft/s (3 m/s). Table 2.1.3.5 (p.11): DN100, 390 gpm (1,475 L/min); DN150, 880 gpm (3,330 L/min); DN200, 1,560 gpm (5,905 L/min); DN250, 2,440 gpm (9,235 L/min); DN300, 3,520 gpm (13,325 L/min).

Documents, Placard and Acceptance Tests

Section 2.6.2.3 (p.152) asks the plan view to show every node in the hydraulic analysis, the design the system is based on, the required flow and pressure and the reference point they are calculated to; it also shows the SIN, K-factor and temperature rating of each sprinkler, linear spacing, distances to walls, the location of heat and smoke vents, and areas where the ambient temperature will fall below 40 °F (4 °C) or exceed 130 °F (54 °C).

Section 2.6.4.6 (p.153) sets a clear rule for the water supply: use flow test data for an existing supply that is no more than 12 months old; for a new fire pump, include the characteristic curve and follow DS 3-7; if a water storage tank is to be installed, meet DS 3-2. Section 2.6.5 (p.153-154) names two forms, FM85A and FM999C; for dry, preaction, refrigerated-area, vacuum or deluge systems, documentation is also submitted showing that the expected water delivery time meets the maximum.

Section 2.2.1.3.5 (p.13) requires a rigid placard at the system valve with at least: the name of the protected area, the hazard, the sprinkler SINs, temperature rating, maximum area spacing, number of sprinklers in the design area, minimum sprinkler design pressure, flow and pressure required at the base of the riser, hose stream allowance and the name of the installing contractor.

For acceptance, Section 2.7.3.2.1 (p.154) requires every new system to be hydrostatically tested at a minimum of 200 psi (13.8 bar), or 50 psi (3.5 bar) above any static pressure exceeding 150 psi (10.3 bar), for 2 hours without pressure loss. Section 2.7.3.2.7 (p.155) adds a pneumatic leakage test of at least 24 hours at 40 psi (2.8 bar) for dry, preaction, refrigerated-area and vacuum systems; leakage causing a loss of more than 1-1/2 psi (0.1 bar) in 24 hours is corrected, and the limit applies to the collective effect, not to individual leaks. Section 2.7.3.1.1 (p.154) uses a full-flow test to confirm that the water supply on site is equal to or greater than that shown on the accepted drawings.

Field Note

Most sizing errors show up not in the formula but in inputs that have gone stale. Section 2.6.4.6 wants the water supply test to be no more than 12 months old; on site, the flow test behind the calculation often comes from the original tender file, and in the meantime a line has changed in the public main. The only check that catches this is the full-flow test of Section 2.7.3.1.1: measure at acceptance and compare with the value on the drawing. The second common error is topology: a project is drawn as a gridded wet system, then one section is converted to dry because it cannot be heated, but the pipe network stays the same. Section 2.2.1.5.2 closes that door; on the dry side, single-path flow is not a preference but the physical precondition for the water delivery time.

Checklist

Frequently Asked Questions

Which Hazen-Williams C-value does FM DS 2-0 recommend?

Where the occupancy-specific data sheet gives no value, use the defaults in DS 2-0 Table 2.2.1.3.3 (Section 2.2.1.3.3, p.13). Wet systems: black steel C=120, polymer enhanced pipe 140, plastic 150. Dry and preaction systems: black steel C=100, but 120 where the gaseous medium is an inert gas such as nitrogen, 120 for internally galvanized pipe and 140 for polymer enhanced pipe. Refrigerated-area and vacuum systems have their own rows.

What is the minimum sprinkler pipe size?

DS 2-0 Section 2.4.1.1.1.1 (p.32) sets the minimum size for sprinkler system piping at 1 in. (25 mm). Section 2.4.1.1.1.2 completes it: whatever size is chosen, it must be hydraulically proven to meet the minimum design recommendations of the relevant occupancy-specific data sheet. Pipe size is not read from a schedule; it is confirmed by calculation.

Can a gridded arrangement be used on any system?

No. DS 2-0 Section 2.2.1.5.1 (p.14) allows tree, loop or grid arrangements for wet-pipe and deluge systems. Section 2.2.1.5.2 requires single-path flow, with no loops or grids, for dry-pipe, preaction, refrigerated-area and vacuum systems. The only exception is Section 2.2.1.5.3: the feed main and cross main of dry-pipe and preaction systems protecting a nonstorage area may be looped.

What limits the size of a dry system?

Water delivery time. Where no value is given, DS 2-0 Section 2.2.1.11.1 (p.16) offers options such as 60 seconds with the single most remote sprinkler operating, or 50 seconds with the most remote 2 sprinklers on the most remote branch line for nonstorage occupancies. Section 2.2.1.11.3 gives the practical limit from experience: to meet these times the system size needs to be limited to approximately 10,000 to 12,000 ft2 (930 to 1,115 m2).

References

  1. FM Global Property Loss Prevention Data Sheet 2-0, Installation Guidelines for Automatic Sprinklers, October 2021, Interim Revision April 2025. Sections used: 2.1.2.3 and 2.1.3.5 (p.10), Table 2.1.3.5 (p.11), 2.1.5 (p.11-12), 2.2.1.3.1-2.2.1.3.5 and Table 2.2.1.3.3 (p.12-13), 2.2.1.4, 2.2.1.5 and 2.2.1.7 (p.14), 2.2.1.11 (p.16), 2.4.1.1.1-2.4.1.1.4 (p.32-34), Table 2.4.1.1.2.1(b) (p.33), Table 2.4.2.2.1(2) (p.36), 2.6.2-2.6.5 (p.152-154), 2.7.3.1-2.7.3.2 (p.154-155).
  2. FM Global data sheet list: https://www.fmglobal.com/research-and-resources/fm-global-data-sheets
  3. DS 2-0 installation side: https://selvi.org/en/blog/fmglobal/ds-2-0-sprinkler-installation
  4. Pipe and hanger detail: https://selvi.org/en/blog/fmglobal/ds-2-0-pipe-and-hanger-detail
  5. Friction loss and C-factor tables: https://selvi.org/en/blog/fmglobal/ds-2-89-pipe-friction-hazen-williams
  6. Hydraulic calculation fundamentals: https://selvi.org/en/blog/fmglobal/ds-3-0-hydraulics-fundamentals
  7. System types compared: https://selvi.org/en/blog/comparison/wet-dry-alternate-preaction-compared

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Standards & References

FM Global Property Loss Prevention Data Sheet 2-0, Installation Guidelines for Automatic Sprinklers, October 2021, Interim Revision April 2025. Every figure in this article is taken from the data sheet text; clause, table and page references are listed under References. General information only: the current edition and the occupancy-specific data sheet govern, and design must be carried out by a qualified engineer.